The recording system of a computed tomography unit which is known, for example, from EP 0 819 406 A1 includes an X-ray machine and a multirow detector. An X-ray machine and detector are arranged lying opposite one another on a rotary frame such that an object volume in the form of a spiral scan can be scanned during rotation of the rotary frame about a rotation axis and during a continuous feed of the object, which is supported on a bearing frame, in the direction of the rotation axis. A tomogram or volumetric image is reconstructed on the basis of the X-ray images acquired by the detector during the spiral scan from different projection directions.
Ring artifacts that are centered about a center of rotation, identifiable in the tomogram, of the recording system of the computed tomography unit cause signal errors of individual detector elements of the multirow detector. DE 198 35 451 A1 describes a method for removing ring artifacts in tomograms produced with the aid of a computed tomography unit. The known method includes method steps for calculating a correction image that substantially includes only the ring artifacts present in the tomogram.
The tomogram is subjected to at least one instance of median filtering for this purpose. An instance of low-pass filtering following thereupon serves the purpose of suppressing interfering noise components and noise structures. The low-pass filtering is performed along circular arcs about the center of rotation in the image. In the case of the known method, a result image in which the ring artifacts are suppressed is produced by a subsequent subtraction of the low-pass-filtered correction image from the input image, it being necessary, however, for the ring artifacts to have a certain minimum length for a successful suppression.
The length of a ring artifact or of a ring segment representing a ring artifact is determined by the speed with which that recording region of an object which is to be scanned is displaced in the direction of the rotation axis during each revolution of the recording system. The ring segment becomes shorter in this case the quicker the feed of the recording region to be scanned. However, ring segments of a short length such as are produced during rapid scanning of the recording region are not acquired or suppressed at all in tomograms in the case of the known method, or only unsatisfactorily so.